Holy lots of different crash issues, batman. Let me see what I can wade through here...
Should not be happening, not enough information provided to additionally diagnose. This should probably get reported at
Mystcraft's BugTracker or their
forum. Sorry I can't be more helpful on this one, but it's really a case of not being able to do more with it without it being right in front of me (there's some wild flailing I could suggest but that may not be constructive; see my response to Slipstream below and substitute "2" for "0" if you want a last ditch thing to try).
Who all are running ops? Are any of those people living in Magnet Mode? If so, can you get them to turn it off and try again? That crash is linked to a check for the NEI Magnet Mode, rather than a direct Mystcraft crash itself.
How many people are you running simultaneously and how much memory are you giving the server? This error is your minecraft server out-and-out running into a memory brick wall and saying "I can't take any more of this crap". If your user limits/memory allocation are reasonable, you may also want to see if there are any world anchors/chunk loaders getting spammed, and limit those.
Another oddball one. World 0 is the overworld; mystcraft shouldn't be messing with that. That said, I had this happen on a private config server once; problem is, all I had to do to fix it was shut the server off and restart it and it behaved again (not very helpful, I know; you obviously had to do the same in loading/unloading mods after all). You
may (and I can't guarantee you'll see this, but it's worth a shot) wish to look in the /world/ directory of your server and see if by chance there's a DIM_MYST0 folder, a DIM0 folder, or if under /world/data there's an agedata_0.dat. If any of those actually exist, get them out of there.
Note: there's a real chance something foofy could have happened to your overworld if this is the case. I make no guarantees; as always, before performing a procedure in the /world/ folder, if you give a damn about your world,
back it up!
If anybody else has suggestions for these folks, they all look to be hairy and relatively obscure issues. Please share your solutions. =)